Studies on DNA replication of plasmid Co1E1 and its relatives have been continued. Initiation of plasmid DNA replication by RNA polymerase, ribonuclease H and DNA polymerase I has been studied. The mechanism of regulation of initiation of replication by a plasmid specific small RNA (RNA I) was elucidated. RNA I inhibits primer formation through interaction with transcripts from the primer promoter. The interaction is enhanced by a plasmid specified protein. Inhibition of primer formation by RNA I determines the copy number of a plasmid in a cell and the incompatibility between related plasmid.